Neutron Spectrometer Based on a Proton Telescope with Electronic Collimation of Recoil Protons

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1 ISSN 7-77, Physics of Particles and Nuclei Letters,, Vol. 9, No. 6 7, pp. 6. Pleiades Publishing, Ltd.,. Original Russian Text V.M. Milkov, Ts.Ts. Panteleev, A.A. Bogdzel, V.N. Shvetsov, S.A. Kutuzov, S.B. Borzakov, P.V. Sedyshev,, published in Pis ma v Zhurnal Fizika Elementarnykh Chastits i Atomnogo Yadra,, No. (7), pp. 7. METHODS OF PHYSICAL EXPERIMENT Neutron Spectrometer Based on a Proton Telescope with Electronic Collimation of Recoil Protons V. M. Milkov a, b, Ts. Ts. Panteleev a, A. A. Bogdzel a, V. N. Shvetsov a, S. A. Kutuzov a, S. B. Borzakov a, and P. V. Sedyshev a a Joint Institute for Nuclear Research, Dubna b The St. Clement of Ohrid University of Sofia Abstract A prototype of a neutron spectrometer based on a gas proportional counter with recoil-proton registration is created at the Frank Laboratory of Neutron Physics at the Joint Institute for Nuclear Research (FLNP JINR) in Dubna. The spectrometer is developed to measure the kinetic energy of protons scattered elastically at small angles that are produced by (n, p) reaction in an environment containing hydrogen. The elaborated prototype consists of two cylindrical proportional counters used as cathodes. They are placed in a gas environment with a common centrally situated anode wire. Studies on the characteristics of the neutron spectrometer were conducted using Cf and 9 Pu Be radioisotope neutron sources. Measurements were made with monoenergetic neutrons produced by the 7 Li(p, n) 7 Be reaction when a thin lithium target was bombarded with a proton beam from an EG- electrostatic accelerator, as well as with neutrons from the reaction D(d, n) He with a gas deuterium target. DOI:./S7776 INTRODUCTION The necessity of measuring the kinetic energy of fast neutrons arose concurrently with their discovery and is still topical. The severity of this problem consists of the absence of an electric charge, resulting in the necessity to transform the neutron energy into the kinetic energy of a charged particle. The mere fact of registration breaks down into two essentially different tasks: () slow thermal neutrons are registered using nuclear reactions with giant interaction cross sections when compared to the typical nuclear interactions; () the second task is, as a rule, associated with measuring the energy distributions of fast neutrons: the results of neutron nuclear interactions characterized by cross sections of no more than ten barn. At resonance energies (from to ~ kev), the time-of-flight method is used, which requires the pulsed operation of a neutron source. The other method is to measure the recoil proton energy in hydrogenous scintillation materials followed by a spectra reconstruction procedure. Proton telescopes that measure the energy of recoil protons in a gaseous medium is a special case for the application of this technique. Some examples are the earlier developed spectrometers presented in papers [ ] and one of the latest works in 6 []. The present paper is devoted to this method of fast neutron spectrometry.. DESCRIPTION OF THE DEVICE A prototype of a neutron spectrometer based on a recoil proton telescope, which employs the method of collimation of these protons, has been developed and created at the Frank Laboratory of Neutron Physics at the Dubna Joint Institute for Nuclear Research (FLNP JINR). The general view of the device is shown in Fig.. The body is made of a thin-walled noncorrosive tube 96 mm in diameter and 6 mm in length. An anode wire (W + Au) with a diameter of μm extends along the whole working section of the detector. The first cathode (cathode A) is at the very beginning of the active volume in the form of a 7-mm-long tube; at a distance of. mm from its edge, the second cathode (cathode B) with a length of mm is mounted. Both electrodes are made of a thin-walled stainless steel tube with an inner diameter of mm. These are supported and centered by means of Teflon rings. The rings themselves are placed on six racks 6. Three signal outputs 7 and a vacuum valve are located on the trailing flange. Figure shows a block diagram of the spectrometer. The anode wire in the body is connected to a 6 A B Fig.. General view of the device. 7

2 NEUTRON SPECTROMETER BASED ON A PROTON TELESCOPE 9 positive high-voltage source ; cathode A and cathode B are connected to a compensating voltage source 6. Signals from the cathodes and anode are passed to the inputs of the charge-sensitive preamplifiers 7. The outputs of the preamplifiers are connected to the inputs of the respective spectrometer amplifiers, whereas their outputs are connected to the inputs of analog-to-digital converters (ADC) 9, which are USB interfaced to a personal computer (PC) via the CAMAC controller. The ADC, the CAMAC controller, and the PC comprise an electronic system that performs the functions of discrimination, the coincidence of anode and cathode signals, and the recording of multidimensional amplitude information. The information gained is processed using the Lada program developed at the FLNP. The multidimensional experimental data processed by the Lada program result in an energy spectrum of neutrons in a given region of a gas target and a given solid scattering angle of recoil protons in the detector.. THE PRINCIPLE OF OPERATION The telescope is built on the basis of a proportional counter with a common anode wire in a hermetically sealed case filled with a hydrogenous gas. The application of two additional electrodes allows one to solve two functional tasks: (i) the creation of a target region in the first tube (cathode A); (ii) the collimation of recoil protons in the solid angle, which is determined by the far end of the second tube (cathode B). The remaining region from the end of the second tube to the end of the detector body makes up a volume in which the recoil protons are stopped. In the detector volume, there are three regions with different radii of cathodes (tubes A and B and the remainder of the counter), which entails a difference in the gas amplification factor. The equalization of the gas amplification factor takes place with the help of feeding the compensation potential to the tubes. In the telescope, collimation happens by way of the coincidence of pulses from recoil protons in both tubes and, subsequently, in the sorting of experimental data using software in the chosen ranges of amplitude distributions of the signals picked off the cathodes A and B. Changing the pulse discrimination threshold in the first tube, one can choose the thickness of the gas layer situated early in the tube, where neutron proton scattering events can occur. Therefore, it is possible to determine the thickness and location of the gas target, as well as to change the solid angle within the limits of which the telescope operates, thus optimizing both the efficiency and the energy resolution of the device Fig.. Block diagram of the spectrometer. The pulse discrimination threshold in the second tube is set near the amplitude peak, which corresponds to the fact of the complete passage of recoil protons through the tube. Since the energy losses are proportional with high accuracy to the path lengths, the thickness of the gas layer of the target may be chosen when setting the discrimination threshold. The ranges of protons in the detector were calculated using the SRIM program []. The time coincidence of the selected pulses from both tubes corresponds to the tracks located completely in the telescope volume. The proton energy depends on the escape angle of the proton with respect to the initial direction of the neutron as follows: E p = E n cos θp. () Registering the protons in a small solid angle, we can get a good energy resolution: de p = E n sin( θ p )dθ p. () Integrating from to θ pmax, we obtain ΔE p = E n ( cosθ pmax ). () It should be noted that the quantity θ pmax depends on the point of track production. A small amount of He is made a component of the gas filling to calibrate energy in the region up to. MeV, as well as to measure the proper resolution of the detector. Distinctive features of the claimed spectrometer are as follows: (i) a gas layer whose thickness and location are arbitrarily chosen during information processing in the electron system is used as a proton target in the first tube, resulting in a considerable decrease in the energy threshold of the registered spectra and in a better resolution; (ii) the second tube serves as a recoil proton collimator (providing that the signals from both tubes coincide), and the minimum collimation angle is chosen after information processing in the electron circuit. PHYSICS OF PARTICLES AND NUCLEI LETTERS Vol. 9 No. 6 7

3 MILKOV et al. Fig.. The online mode.. SOFTWARE In the present paper we use a few programs for computations, analysis, and the simulation of processes in the detector. The SRIM program helps determine ionization losses and range lengths in gases (the Slowing Down and Range of Ions in Targets package). We also use the Lada program for the collection, visualization, and analysis of experimental data. Below we briefly review these programs... SRIM SRIM is a set of software packages that compute the multitude of characteristics of ion transport in materials. The most characteristic applications are the following []: (i) slowing down and ranges of ions in targets. SRIM is used to calculate most aspects of energy losses and ion slowing down in materials; it is also possible to perform fast calculations and obtain tabulated data on the slowing-down power, the deflection of ion tracks from the initial direction of motion (straggling), and the relative energy losses for all ions and media in a wide energy range; (ii) ion implantation; (iii) ion sputtering; (iv) ion therapy. In the present study we employ the Slowing-Down and Range of Ions package to determine proton ranges for the given gaseous mixture and detector geometry... Lada Lada was written for the operative control over physics data collection with the possibility of the data being recorded to media during an experiment. The package allows one to visualize up to four one-dimensional energy and time spectra and carries out a detailed view of them. It is also used for equipment adjustment. In the offline mode, it is possible to construct integral spectra by multiparameter criteria from the stored initial (raw) information. In the online mode, the data reach the PC memory in list mode and are accepted by the program via one of the USB channels (Fig. ). An array of consecutive buffers is set up for equalizing the peak loads. The entered information is analyzed and processed by an individual thread, the event is identified by three higher bits, service bites are removed, and error conditions are corrected. The thread is activated to refresh the spectra either during the filling of the output buffer or intervals of time. The recording of the source data to files is the ping-pong mode in binary form. The integral spectra are saved in ASCII codes. It is possible to view the sectors of each spectrum in detail and process peaks and chosen regions. A built-in generator, which simulates the accumulation process and enables one to do without a real physics apparatus, is provided for checking the program algorithms and training in the basic actions. In the offline mode it is possible to prepare integral spectra from binary files with the stored source data or view the recorded ones (Fig. ). When operating with binary files, it becomes possible to sort events connected with each other by specific criteria. PHYSICS OF PARTICLES AND NUCLEI LETTERS Vol. 9 No. 6 7

4 NEUTRON SPECTROMETER BASED ON A PROTON TELESCOPE Fig.. The offline mode. This relates to recording the events into the spectrum of window only provided that the related data are brought into the separate spectra regions of windows and. Conversely, when a spectrum sector of window is picked out, the data for the selected region are processed and displayed in windows and.. MONTE CARLO SIMULATION A simulation using the Monte Carlo method was performed to study the characteristics of the proton telescope. Fast neutrons are scattered by a hydrogenous gaseous target, and the recoil protons arrive at a cylindrical counter with a radius of R c and length of L. The neutrons registered by the counter are emitted parallel to the axis z. Assume that a recoil proton emerged at a point with coordinate z at distance r from the cylinder axis and travels in the direction defined by angle θ with respect to the cylinder axis and by angle ϕ measured from the vertical direction (Fig. ). Then the proton crosses the cylinder boundary at the distance r r ϕ r = cos + cos ϕ + R c r. () The distance covered by the proton in the cylinder is equal to If the proton appeared at a point with coordinate z, then it crosses the cylinder boundary at a point with coordinate z = z + l p cosθ. (6) The coordinate z may not exceed the maximum counter length. The program simulates proton tracks in relation to escape angles θ and ϕ. The track origin with coordinate z and the distance r from the counter symmetry axis are developed. Then the angles θ and jϕ are developed and the length of the track up to the point of its intersection with the tube is calculated. The values of the angles and original coordinate varied in the following limits: r R c, θ π -, ϕ π, z z max, For solid-angle estimation, events are selected in which the protons have completely crossed the second tube. Therefore, the events corresponding to the pro- n z z max L =.9 cm p θ θ π/, z z max, r R c, ϕ π z r ϕ R c r l p = r sinθ () Fig.. Calculation of the recoil proton spectrum in a cylindrical counter. PHYSICS OF PARTICLES AND NUCLEI LETTERS Vol. 9 No. 6 7

5 MILKOV et al., Amplitude 7 6 Gas: mbar CH + mbar Ar + mbar He E n = 9 kev Fig. 6. Calculated forms of proton spectra for various track production regions: () < z <. cm, () < z <. cm, and ()< z < 7. cm. Experimental results of monoenergetic neutron fluxes of E = 9 kev obtained using the EG- electrostatic generator at FLNP JINR. ton tracks in the small solid angle along the detector symmetry axis are taken into account, allowing for a high-resolution determination of the energy of an incident neutron. Computations were performed for monoenergetic neutrons of energy MeV. The track length is 7 cm, and the maximum distance from the origin of the first tube to the end of the second one is L =.9 cm. The tube radius is R c =. cm. Three versions of calculations were made: () < z <. cm, () < z <. cm, and () < z < 7. cm. Every time, 7 events were developed. The result was that the width of the peaks increases with an increasing proton production region (Fig. 6a). A fraction of the measured spectrum is shown for comparison in Fig. 6b. The measurement conditions are described below. The efficiency was estimated by the formula ΔΩ ε = n H σ np , (7) π which is valid for a small gas concentration and takes into account the fact that the protons move forth. n H is the number of hydrogen nuclei per cm in the detector; σ np is the scattering cross section. CH gas was utilized under a pressure of mbar: N H n H = =. 9 cm. () S Using the simulation result ΔΩ/π., we find that the efficiency of the telescope at a pressure of mbar is ε ( MeV).6 6. The magnitude of the telescope efficiency is linearly dependent on the pressure of the working gas.. MEASUREMENTS To test the functionality of the detector, the following experiments were conducted: an α source of Am was fixed to the origin end of the first tube; the pressure in the detector volume was. atm, which excluded the possibility that α particles from the first tube volume (cathode A) are brought into the second tube volume (cathode B). The detector was also exposed to thermal neutrons after the Cf moderator. Figures 7 Neutron sourse Gf, Alpha sourse Am Gas:.6 bar (% Ar + %CH + % He) Neutron sourse Gf Gas:.6 bar(% Ar + %CH + % He) kev kev Am Fig. 7. Spectrum from cathode A. Fig.. Spectrum from cathode B. PHYSICS OF PARTICLES AND NUCLEI LETTERS Vol. 9 No. 6 7

6 NEUTRON SPECTROMETER BASED ON A PROTON TELESCOPE 6 Spectrum from anode Neutron sourse Cf Gas: mbar Ar + mbar CH + 7 mbar He HV anode = 7 V Compensatory voltage for cathodes = V A Tritium 9 kev 6 Spectrum from anode Neutron sourse Cf Gas: mbar (Ar % + CH %) + mbar He FWHM ~. % Peak from He with full energy kev Proton 7 kev He kev B Fig. 9. Spectra from the reaction He(n,p)T kev; spectrum A corresponds to a measurement without compensatory potential and spectrum B corresponds to one with compensation for the electric field. Fig.. Proper energy resolution with pulses from both tubes coincident. The spectrum from the reaction He(n, p)t kev from the Cf source; the moderator is a polyethylene block. and show the results of measurements. It is clear that there is no effect of the α source on the spectrum of the second cathode tube, the positions of the peak from He in both tubes are exactly the same, and the asymmetry of the shape in the second cathode is a result of the location of the thermal neutron source. Figure 9 shows the measurement results of anode spectra from the reaction He(n,p)T kev. The combination of two peaks, i.e., the equalizing of the gas amplification factors over the whole detector, is achieved by feeding the positive potential to the cathode tubes U k, the magnitude of which is found by trial and error. The addition of a small amount of He to the gaseous mixture allows one to experimentally measure two extremely important parameters of the proton telescope. () The proper energy resolution of the detector was measured using a standard Cf neutron source and a polyethylene moderator block. It was ~.% (FWHM). The results are presented in Fig.. () The peak of full absorption of the proton and triton energy in the detector volume allows a highaccuracy energy calibration up to the recoil proton energy MeV. It is also possible to use the escape peaks of the proton (7 kev) and triton (9 kev) for telescope calibration at low neutron-flux energies. A series of measurements of monoenergetic neutron fluxes produced in the reactions 7 Li(p,n) 7 Be and D(d,n) He were carried out at the FLNP EG- electrostatic generator. Figure shows the spectra of monoenergetic neutrons with energies from to 9 kev obtained from the thin 7 Li target after software processing. Based on these results, the target thickness was estimated to be about kev. A gaseous mixture mbar CH +.9 mbar Ar + mbar He was utilized in the measurements. In Fig. b, the result of a comparison of experimental neutron spectra data according to the detector calibration (black dots) is shown with data on the energies of the proton beam from the EG- accelerator (grey dots). The discrepancy in the low energy range is mainly due to noticeably more nonlinear energy losses of protons within the thickness of the lithium target. This discrepancy also includes a proton-beam energy spread downstream of the turning magnet, the field stability of which was determined by the paramagnetic resonance frequency of the hydrogen detector being measured. The standard deviation of the points from the straight line is no more than %. Measurements in the low-energy region of the neutron spectra were performed by means of the 7 Li target, and the lower boundary of the spectrometer was determined. The measurement results without software processing for the energies and kev are shown in Figs. a and b. These results after processing are presented in Fig. c. According to the experimental data, the energy resolution of the spectrometer is from 7 %. This wide region is primarily due to the straggling effect (the deflection of the recoil proton tracks from the initial direction, especially at the end of the ranges). This PHYSICS OF PARTICLES AND NUCLEI LETTERS Vol. 9 No. 6 7

7 MILKOV et al. Gas: mbar CH +.9 mbar Ar + mbar He 6 kev 69 kev kev 7 kev 79 kev 7 kev 9 kev 6 E, kev Result from teleskope Result from EG Fig.. Spectra from monoenergetic neutrons of energies from to 9 kev obtained from the thin 7 Li target after software processing. Result of a comparison of the experimental data on neutron spectra according to detector calibration (black dots) with data on the proton beam energy from the EG- accelerator (grey dots). 7 6 Anode HV anode = V HV cathode A and B = 97 V E n = kev Gas: Ar + CH + He = 6 mbar kev kev (c) 7 9 Anode HV anode = V HV cathode A and B = V E n = kev Gas: Ar + CH + He = 6 mbar Fig.. Results of measurements without software processing for the energies kev and kev. (c) Results upon processing for the energies and kev. results in considerable energy losses by the so-called wall effect (the arrival of recoil protons at the walls and structural materials). Straggling in combination with the nonlinearity of the quantity de/dx for protons during their slowing-down significantly hinders the choice of the range of spectra integration in the second tube. This requires the application of the matrix technique of data acquisition and processing. On the other hand, electron collimation makes it possible to bring (especially for monoenergetic fluxes) the resolution of the spectrometer closer to its proper one. A series of experiments were conducted with a gas deuterium target in an effort to extend the energy range of measurements: points in the range from. to. MeV were measured. A gaseous mixture of 9 mbar CH + mbar He was used in the measurements. Some of the spectra before and after sorting are shown in Fig.. The energy resolution is within %. Figure shows a channel energy relationship for the measured points; we observe a good linear dependence of energy on the number of the amplitude PHYSICS OF PARTICLES AND NUCLEI LETTERS Vol. 9 No. 6 7

8 NEUTRON SPECTROMETER BASED ON A PROTON TELESCOPE MeV. MeV.76 MeV MeV. MeV. MeV 6. MeV 6.76 MeV MeV. MeV E, MeV E, MeV Results from telescope Fig.. Channel energy relationship for the measured points (energies) from the reaction D(d,n) He in the range from. to. MeV. Gaseous mixture 9 mbar CH + mbar He. 96 Fig.. Some of the spectra before and after sorting in the range from. to. MeV (the reaction D(d,n) He). Gaseous mixture 9 mbar CH + mbar He. analyzer channel. The standard deviation of the points from the straight line is no more than %. A 9 Pu Be neutron source with a known spectrum was measured. A portion of this spectrum was reconstructed. Data on the energy distribution of the neutron flux, which were obtained by scintillation stilbene detectors and by the emulsion method [6], are presented in Fig. a. Results for three energy ranges (.. MeV,.. MeV, and. 6. MeV) obtained with the proton telescope are shown in Figs. b d. The full absorption energy from the reaction He(n,p)T kev (whereby the spectra are calibrated) is marked by a line at the beginning of each spectrum. These measurements show a material drawback of the device: the small dynamic range of energy (E max /E min = ). 6. CONCLUSIONS In the present paper we described the design of the spectrometer and the electronics used. We have considered the principle of operation and application of the electron collimation method. The detector characteristics (the effective solid angle and the dependence of the resolution on the gas target thickness) have been studied by simulating the processes in the detector; the results are given in Fig. 6a; a comparison with experimental data is shown in Fig. 6b. The resolution becomes poorer as the thickness of the gas layer of the target is increased. The detector efficiency computed for the working gas CH at a pressure of mbar and neutron energy PHYSICS OF PARTICLES AND NUCLEI LETTERS Vol. 9 No. 6 7

9 6 MILKOV et al. Relative intensity Neutron energy, MeV. MeV kev Sourse 9 Pu Be Gas: mbar CH + He. MeV Stilbene data Emulsion data. MeV. MeV kev. MeV kev Sourse 9 Pu Be Gas: 9 mbar CH + He MeV. MeV Sourse 9 Pu Be Gas: 7 mbar CH + He. MeV 6. MeV (c) (d) Fig.. Results of the neutron-beam energy distribution obtained by scintillation stilbene detectors and by the emulsion technique [6]. (b d) Data obtained with the proton telescope for the energy ranges.. MeV, (c).. MeV, and (d). 6. MeV. of MeV is 6. It should be kept in mind that this quantity is a function of the gas pressure, the chosen degree of electron collimation, and the geometrical parameters of the telescopes. Formula (7) makes it possible, to a certain degree, to estimate this parameter. To check the capacity of the device for work, spectra from the first and second cathodes (electrodes) have been recorded and a measurement with the use of an alpha source has been performed which confirmed the linear energy dependence of the pulse amplitude on the energy released in the working gas and absolute independence of the operation of each cathode. The results obtained are fully satisfactory. The device was adjusted, and was been shown by means of the compensatory potential fed to the cathodes that the energy resolution remains unchanged. Given the coincidence of pulses from the protons and tritons from the reaction He(n,p)T in both cathode tubes, the properly chosen potential has shown that the resolution is approximately equal to.%. Calibration in the range 9 kev was done based on the measurements with monoenergetic neutrons emitted by the 7 Li target in the electrostatic generator EG- at FLNP. The lower limit of the telescope operation is kev. Measurements with the use of a gaseous deuterium target were performed; the calibration range of EG- operation was extended to. MeV. The energy resolution for a fast neutron from the 7 Li target and gaseous deuterium target is within the range 7 %. The upper limit of measurements was extended to 6. MeV through measurements with the 9 Pu Be source. It should be noted that the overfilling of the detector with methane was due to the small energy range of operation and the ranges of recoil protons had to be recalculated. Despite this, it may be unambiguously concluded that the spectrometer can be successfully used to measure the parameters of quasi-monoenergetic neutron fluxes. REFERENCES. K. Asai, N. Naoi, T. Igughi, K. Watanabe, J. Kawarabayashi, and T. Nishittani, Neutron Spectrometer for DD/DT Burning Ratio Measurement in Fusion Experimental Reactor, Nucl. Sci. Tech., (6).. H. Borst, Proportional Counter Telescopes for Fast Neutron Spectrometry, Nucl. Instrum. Methods Phys. Res. 69, 69 7 (9).. M. Mizuho, A Gas Recoil Fast Neutron Spectrometer, Nucl. Instrum. Methods Phys. Res. 7, 9 (969).. M. Mizuho and T. Yamanaka, A Fast Neutron Spectrometer with Fast Coincidence Technique in Proportional Counters, Nucl. Instrum. Methods Phys. Res. 9, 7 6 (97).. James Ziegler - SRIM & TRIM. SRIM 6. M. E. Anderson and R. A. Neff, Neutron Energy Spectra of Different Size 9 Pu-Be (α, n) Sources, Nucl. Instrum. Methods Phys. Res. 99, (97). PHYSICS OF PARTICLES AND NUCLEI LETTERS Vol. 9 No. 6 7